A variety of electrical equipment contain switches which interrupt or direct the path of electricity through an electric circuit. Circuit breakers, for example, are switches used to open a circuit in the event of a fault, or short circuit. Another type of switch is a load tap changer, which is used to automatically select a particular tap corresponding to a connection within the secondary windings of a transformer.
The parts of a switch which actually perform the function of connecting and disconnecting the current path are called contacts. In high voltage equipment, the contacts of electrical switches operating under load generally erode over time during operation. The erosion of electrical contacts most commonly results from the arcing that occurs whenever a switch breaks, or interrupts, a circuit. An arc is formed as the electrical contacts move apart from or toward each other and the electric potential between them causes electrons to bridge the inter-contact space region. A current is maintained in the arc until the spacing between the contacts, and thus the impedance, increases enough to prevent electrons from bridging the gap for the given voltage potential, or, if moving toward each other, until the contacts are touching. The current flowing across the gap generates extreme heat, resulting in temperatures high enough to burn away some of the contact material.
Erosion of the contacts can cause failures or deteriorated switch operation and otherwise generally reduce or limit the useful lives of the switches themselves. Switches may fail when their contacts have eroded to such a degree that they cannot effectively complete a circuit, or when the erosion has changed the physical shape of the contact such that the mechanical operation of the switch is interrupted. Once a contact has eroded to the point at which further use risks injury to personnel or machinery, known as the “critical point,” a contact's useful life is over.
Because arcing and erosion cannot be eliminated, switches are almost always designed to allow replacement of the contacts. It is typically less expensive to replace worn contacts than to replace an entire switch when the contacts have eroded to the critical point or close thereto. As a result, users of switches must monitor the erosion of the contacts to recognize when the predetermined critical point is approaching or has been reached. Replacing worn contacts at or before the critical point is important because contacts used past that point continue to erode and may cause the switch to fail. A switch failure can have a negative or catastrophic effect on equipment and presents a danger to personnel. On the other hand, replacing contacts before the end of their useful life increases material and labor costs.
There is a large expense associated with electrically isolating, or de-energizing, and physically inspecting high voltage electrical equipment to determine the extent of wear or erosion of the contacts. This expense is compounded by the necessity of removing, storing, and processing a large quantity of oil, sometimes up to 1000 gallons. Contacts are often replaced early due to the difficulty of predicting the rate of erosion from one maintenance cycle to the next. The expense of inspecting the contacts is often so great that many times maintenance departments change some of the contacts during every inspection, even though the contacts may have months or more of useful life remaining. Properly matching the timing of inspection with the end of the useful life of the contacts would thus advantageously result in a cost savings.
One means commonly used to monitor electrical equipment performance and identify equipment requiring maintenance is the use of dissolved gas analysis (DGA). In DGA, a sample of the oil surrounding the contacts is extracted and analyzed to monitor for dissolved gases. The presence of certain gases is indicative of various types of problems that may be occurring within the equipment. For example, the presence of acetylene dissolved in the oil surroundings can be indicative of excessive heating within load tap changers and transformers. The DGA method of monitoring, however, lacks the precision necessary to determine the proper timing of contact replacement, as the presence of dissolved gases related to erosion has no correlation to the amount or extent of erosion of the contacts.
There is accordingly a need to provide a method and apparatus for the detection of the extent of electrical contact erosion, or wearing, that is inexpensive and may be used on-site as well as in the laboratory.